1. Field of the Invention
The invention relates to a sewing machine capable of forming a buttonhole, and more particularly, to a sewing machine capable of shortening a cycle time for forming a buttonhole.
2. Description of Related Art
FIGS. 2, 3, and 4 show the structure of a buttonhole sewing machine. A frame 9 comprises a bed 12, a pedestal portion 10 and an arm 11. The pedestal portion 10 is mounted on the bed 12. The arm 11 extends from the pedestal portion 10 so as to be almost parallel to a bed side 12A of the bed 12. A hammer 4 is provided on the left side of the pedestal portion 10. The hammer 4 cooperates with a cutter blade 3 to form a buttonhole slit, such as a straight hole slit or an eyelet hole slit on the work fabric 7. The eyelet hole slit consists of a round portion and a straight portion. A needle bar 6 is provided under the head portion of the arm 11. The needle bar 6 forms buttonhole stitches around the periphery of the buttonhole slit in order to prevent threads from coming loose. The needle bar 6 has a needle 5 therein.
On the bed 12, there is provided a feed table 2 on which the work fabric 7 is set. The feed table 2 can move horizontally with the work fabric 7 so as to be almost parallel to the bed side 12A. A pressure foot 8 for holding the work fabric 7 is provided on the feed table 2. Under the feed table 2, a looper 1 and the cutter blade 3 are fixed to the bed 12. The looper 1 forms buttonhole stitches in cooperation with the needle 5. The cutter blade 3 is used as a lower knife and for preventing the work fabric 7 from getting out of position.
The operation of a conventional buttonhole sewing machine will now be described. Conventionally, a pre-buttonhole slit method and a post-buttonhole slit method are known. According to the pre-buttonhole slit method, buttonhole stitches are formed after the buttonhole slit is formed on the work fabric 7. On the other hand, according to the post-buttonhole slit method, the buttonhole slit is formed after buttonhole stitches are formed on the work fabric 7. Both methods can be selected in the conventional buttonhole sewing machine. Therefore, a user selects either the pre-buttonhole slit method or the post-buttonhole slit method based on the material of the work fabric 7.
FIG. 6 is a time chart for showing the operation of the pre-buttonhole slit method.
The user removes the processed work fabric and sets a non-processed work fabric 7 immediately under the hammer 4. A section AB of the time chart shows this operation time. When a starting switch is turned on, the hammer 4 comes in contact with the cutter blade 3 through the work fabric 7. The hammer 4 cooperates with the cutter blade 3 to form a cutout on the work fabric 7 as the buttonhole slit. A section CD of the time chart shows this operation time. The shape of each buttonhole slit is determined to be either the straight hole slit, or the eyelet hole slit whose one end has a round portion, according to the shape of the cutter blade 3. The feed table 2 moves with the work fabric 7 and the buttonhole slit in the work fabric 7 is placed under the needle 5. A section DE of the time chart shows this operation time.
Next, the buttonhole stitches are executed. The needle 5 and the looper 1 are simultaneously driven. As a result, the buttonhole stitches are formed around the periphery of the buttonhole slit on the work fabric 7. A section EF of the time chart shows this operation time. When the buttonhole stitches are completed, the feed table 2 moves with the work fabric 7 to a position under the hammer 4. A section FG of the time chart shows this operation time. Thus, the operation of the pre-buttonhole slit method is completed. The buttonhole sewing machine is ready for the user to remove the processed work fabric 7 and set a non-processed work fabric 7.
FIG. 7 is a time chart for showing the operation of the post-buttonhole slit method of the conventional buttonhole sewing machine. The user removes the processed work fabric and sets a non-processed work fabric 7 under the hammer 4. A section HI of the time chart shows this operation time. When the starting switch is turned on, the feed table 2 moves with the work fabric 7 to under the needle 5 where the buttonhole slit will be formed on the work fabric 7. A section JK of the time chart shows this operation time.
Next, the needle 5 and the looper 1 synchronously move in order to form buttonhole stitches. At that time, the feed table 2 moves with the work fabric 7 in synchronization with the movement of the needle 5 and the looper 1 so that buttonhole stitches are formed around the periphery of the place where the buttonhole slit will be formed. A section KL of the time chart shows this operation time. When the buttonhole stitches are completed, the feed table 2 moves with the work fabric 7 until the buttonhole stitches on the work fabric 7 are positioned under the hammer 4. A section LM of the time chart shows this operation time. Afterwards, the hammer 4 comes in contact with the cutter blade 3 through the work fabric 7 and between the buttonholes stitched defining the buttonhole. The hammer 4 cooperates with the cutter blade 3 to form a cutout in the work fabric 7 as the buttonhole slit. The shape of buttonhole slit is set in either the straight hole slit or the eyelet hole slit whose one end has a round portion. A section HN of the time chart shows this operation time. Thus, the operation of the post-buttonhole slit method is completed and the buttonhole sewing machine is ready for the user to remove the processed work fabric 7 and set a non-processed work fabric 7.
As described above, in both methods of the pre-buttonhole slit method and the post-buttonhole slit method, the user has to set the work fabric 7 under the hammer 4.
However, in the post-buttonhole slit method, there is technically no need for the user to set the work fabric 7 when the feed table 2 is under the hammer 4 as the first action is sewing the buttonhole. Further, it is troublesome for the user to set the work fabric 7 right under the hammer 4, because the user has to set the work fabric 7 while avoiding the needle 5.
In the post-buttonhole slit method, the feed table 2 moves from under the hammer 4 to right under the needle 5 by receiving the starting signal after the user sets the work fabric 7. Therefore, the cycle time for the series of operations of the post-buttonhole slit method is long. That is, the cycle time of the pre-buttonhole slit method is section AG, shown in FIG. 6, and the cycle time of the post-buttonhole slit method is section HN, shown in FIG. 7. It appears that the cycle time of the pre-buttonhole slit method is as long as that of the post-buttonhole slit method. However, in the pre-buttonhole slit method, the user can remove the processed work fabric 7 while the work fabric 7 is moving with the feed table 2 in section FG as the buttonhole has been fully formed. Therefore, for all practical purposes, the cycle time of the post-buttonhole slit method is longer than that of the pre-buttonhole slit method.
Moreover, since the user has to accurately set the work fabric 7 under the hammer 4 while avoiding the needle 5, the work efficiency for forming the buttonhole is reduced.